Brushless dc motor having automatic braking

ABSTRACT

A single-sensor, two-winding brushless DC motor includes a main power supply from which the stator windings are energized and an auxiliary power supply through which a commutating circuit is energized. A position-sensing circuit provides a command signal only while the rotor is within a given 180* sector. The commutating means completes a circuit through a first stator winding in response to a command signal; a circuit through a second stator winding in the absence of a command signal. Dynamic braking is achieved by turning off the main power supply while keeping the auxiliary power supply on. Under these conditions, the commutating means completes a circuit through the second stator winding so that the kinetic energy of the rotor may be dissipated and the motor stopped smoothly and rapidly.

United States Patent BRUSHLESS DC MOTOR HAVING AUTOMATIC BRAKING PrimaryExaminerG. R. Simmons Attorney-S. C. Yeaton ABSTRACT: A single-sensor,two-winding brushless DC motor includes a main power supply from whichthe stator windings are energized and an auxiliary power supply through7 Claims 2 Drum" Figs which a commutating circuit is energized. Aposition-sensing US. Cl 318/25, circuit provides a command signal onlywhile the rotor is 318/ 7, 8/ within a given 180] sector. Thecommutating means completes Int. Cl. 1102K 29/00 a circuit through a,first stator winding in response to a com- Field of Search 318/254, mandsignal; a circuit through a second stator winding in the 133, 364absence of a command signal. Dynamic braking is achieved by turning offthe main power supply while keeping the auxiliary References powersupply on. Under these conditions, the commutating UNITED STATE PATENTSmeans completes a circuit through the second stator winding 3,374,4103/1968 Cronquist m1. 318/685 so that the kinetic energy of the rotor maybe dissipated and 3 1954 I Van Emden v 7 318/138 I h motor pp smoothlyand p y- TO IV P C A R [9 SUPPLY 143 5 7 -49 2, [35 AulfilARY COMMANDPOWER SUPPLY mmm 4m OFF ON SUPPLY AUXILIARY POWER SUPPLY I! l I9 ii 37 v23 29 COMMAND '17 1 7.; COMMUTATING 1 5; 1 CIRCUIT w a!" N I9 s r\ I 25FIG 1 To +v MAIN 45 POWER SUPPLY 43 57 I9 49 2! 35 TO I AUXILIARY 2/POWER SUPPLY COMMAND 6/ INVENTOR. Poa/vfr 6 Emma FIGZ.

BY MT! ATTORNEY BRUSI-ILESS DC MOTOR HAVING AUTOMATIC BRAKING BACKGROUNDOF THE INVENTION 1. Field of the Invention The invention relates tobrushless DC motors and more specifically to braking means for brushlessDC motors.

2. Description of the Prior Art A wide variety of brushless DC motors(BDCMs) is known in the art. In one type of BDCM, a single sensor isused to detect the semicircular sector which the rotor occupies at anygiven instant. Stator windings are energized in accordance with thisinformation.

U.S. PAT. NO. 3,493,831 issued to A. S. Roberts, Sr., and assigned tothe present assignee, for instance, concerns a single-sensor BDCM havinga single pair of stator poles and a sensor of the aforementioned type.Each stator pole is wound with first and second coils. The first ofthese coils on each pole are serially connected so as to produce amagnetic field of one sense whereas the second coils are seriallyconnected so as to produce a magnetic field of the opposite sense. Thusa given stator winding consists of two coils. A commutating meansalternately energizes the first and second windings in response toswitching signals from the sensing means.

The value of such motors lies in their simplicity and dependability.Prior art braking systems require logic circuits and complex circuitry.The braking system of the present invention provides braking with arelatively simple circuit in keeping with the overall simplicity of thebasic motor.

SUMMARY OF THE INVENTION Dynamic braking capability is provided in asingle-sensor, two-winding BDCM by employing separately energizedcommutating means that operate to connect one of the stator windings ina closed circuit in the absence of a signal from the single sensor.Braking is achieved by terminating the flow of external power to thestator windings while maintaining the flow of power to the commutatingmeans. Under these conditions, a stator winding is connected in a closedcircuit so that the kinetic energy of the rotor may be absorbed byvirtue of currents flowing in the closed circuit as a result of voltagesinduced in the stator winding by the moving magnetic field associatedwith the rotor.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view, partly inperspective, functionally illustrating the mechanical features of atypical motor employing the principles of the invention, and

FIG. 2 is a diagram of a circuit that may be used in practicing theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a typicalmotor employing the principles of the invention. The motor consists of ahousing 11 inside of which a permanently magnetized rotor 13 may rotate.A pair of stator poles l and 17 are arranged along a diameter of thehousing. Each stator pole carries two oppositely wound coils.

Thus the stator pole 17 is wound with the coils 19 and 19' whereas thepole is wound with the coils 21 and 21. The coils 19 and 21 areconnected in series through a lead 23 whereas the coils 19 and 21 areconnected in series through a lead 25. A commutating circuit, which willbe explained in' The rotor 13 is mounted on a shaft 29 which alsocarries a semicircular shutter 31. The shutter rotates between a lightsource 33 and a photosensor 35. Thus the photosensor 35 is illuminatedduring one-half of each revolution under normal conditions. In this way,when the rotor is aligned so that its N-S axis is anywhere within agiven semicircular sector, an output will be derived from thephotosensor 35. This output constitutes a command signal. When the rotoris aligned within the opposite sector, the photosensor 35 is darkenedand there is no command signal.

The stator windings, as well as the light source 33 and the photosensor35, are energized from the main power supply 27.

A commutating circuit 37 receives the information from the photosensor35 and switches the appropriate stator winding into the circuit as willbe explained. The commutating circuit is energized from a separateauxiliary power supply 39.

The main power supply 27 may be turned off independently of theauxiliary power supply by any convenient means as functionallyillustrated by the switch 41.

As will be evident to those skilled in the art, the positionsensingmeans comprising the shutter 31, the light source 33 and the photosensor35 operate in conjunction with the commutating circuit 37 to energizethe stator windings in synchronism with rotor rotation so as to attractthe magnetic poles on the rotor at appropriate times so as to maintainrotation.

The operation of the motor and the braking circuit may be betterunderstood by referring to FIG. 2. The positive side of the main powersupply is applied to a light source 33. This light source mayconveniently take the form of a light-emitting diode, an incandescentlamp, or other conventional source. A series dropping resistor 43 may beconnected in series with the light source 33, if desired.

A first stator winding 45, which includes one of the coils on eachstator pole, is connected to one side of the main power supply andthrough a transistor 47 to the other side of the power supply.Similarly, a second stator winding 49, which includes the other coil oneach stator pole, is connected to the high-voltage side of the mainpower supply, and through a transistor 51 to the other side of the mainpower supply. A diode 53 is connected across the transistor 47 and adiode 55 is connected across the transistor 51. A photosensor 35 isconnected through a suitable dropping resistor 57 to the high voltageside of the main power supply.

The output of the photosensor 35 is connected to a first inverter 59which, in turn, is connected to a second inverter 61. The inverters areenergized from the auxiliary power supply 39. The output of the firstinverter 59 is coupled to the base electrode of the transistor 47through a suitable dropping resistor and the output of the secondinverter is coupled to the base electrode of the transistor 51 throughanother suitable dropping resistor.

In operation, both the main power supply and the auxiliary power supplyare energized while the motor is running normally. During the portion ofa revolution when the photosensor is illuminated, a command signal willbe applied to the inverter 59. This will cause the output of theinverter 59 to drop to a low level and turn off the transistor 47. Thelow-level output signal of the inverter 59 is also applied to the secondinverter 61. This causes a high level signal to appear at the output ofthe inverter 61 so as to turn on the transistor 51.

Under these conditions, an energizing current will flow through thestator winding 49 so as to cause rotation of the rotor.

When the rotation has progressed sufficiently far so that the shutter 31prevents light from reaching the photosensor 35, the command signal fromthe photosensor will disappear. This will cause a high-level signal toappear at the output of the inverter 59 and a low-level signal to appearat the output of the inverter 61. The transistor 47 will now be driveninto conduction and the transistor 51 will be cut off.

Energizing current will now flow through the stator winding 45 and amagnetic field will appear between the poles of the motor. This willcause continued rotation of the rotor.

It will be noticed that the inverters 59 and 61 operate as a bistabledevice having a first stable state in which the transistor 47 is driveninto conduction and a second stable state in which the transistor 51 isdriven into conduction.

Thus as long as the auxiliary power supply 39 is energized, one or theother of the transistors 47 and 51 will be in a conducting state.Therefore, even though the main power supply is turned off so that thephotosensor 35 is not energized, a high-level signal will be produced bythe inverter 59 and the transistor 47 will be in a conductive condition.

Assume now, that it is desired to brake the motor after it has beenrunning normally. The main supply voltage will be turned off while theauxiliary supply voltage is kept on. This will turn on the transistor47. The momentum of the rotor will cause it to keep rotating and themagnetic field associated with the rotor will induce voltages in thestator winding 45. Currents resulting from this induced voltage can flowthrough the transistor 47, the diode 55 and the stator winding 49 in aclosed loop. This current will be absorbed in the circuitand act as adrag on the rotor. In accordance with well-known principles of dynamicbraking, the rotor will now come smoothly and rapidly to a halt.

In summary, the motor may be run in its normal condition by applyingvoltages from the main power supply and the auxiliary power supply.Dynamic braking may be achieved by turning off the main power supplywhile keeping the auxiliary power supply on. If desired, the motor maybe allowed to coast to a stop by simultaneously turning off both themain power supply and the auxiliary power supply. Alternatively, themotor may be allowed to coast to a stop by turning the auxiliary powersupply off before the main power supply is turned off.

The diodes 53 and 55 serve to suppress transients across the transistors47 and 51.

The bistable means including the two inverters 59 and 61 mayconveniently take the form of a miniaturized circuit as is well known inthe art.

The transistors 47 and 51 act as switching means and could be replacedby any suitable alternative switching means. Each of the transistors forinstance could be replaced by a cascaded series or a conventionalDarlington amplifier if more gain is required. The light source may beconnected in shunt as shown; however, in some cases, it may be desirableto connect the light source in a known type of series circuit betweenthe main power supply and the commutator circuit.

The circuit diagram of the present invention illustrates how a motoremploying the principles of the invention may be constructed in a simpleand reliable form. Miniaturization may be used to reduce the size ofthemotor circuits to a minimum.

It will be appreciated that the motor may be made reversible if desiredby simple expedients such as incorporating a doublepole, double-throwswitch in the lines leading from the commutating circuit to the statorwinding.

In such a form, a motor is particularly useful in applications where themotor is to drive a load that must be reversed quickly. In certaincamera-aiming operations, for instance, the motor may be required tooperate the camera in a scanning operation. The dynamic braking assuresthat the camera will be brought to a smooth and rapid stop at the end ofeach scan.

While the invention has been described in its preferred embodiments, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withinthe purview of the appended claims without departing from the true scopeand spirit of the invention in its broader aspects.

I claim:

1. A single-sensor brushless DC motor comprising a cylindrical housing,first and second stator windings arranged to establish magnetic fieldsin opposite directions along a diameter of the housing, a permanentlymagnetized rotor rotatable within said housing, a main power supply forenergizing said stator windings, sensing means energized by said mainpower supply for providing a command signal when and only when the rotoris aligned with a given semicircular sector, commutatlon means includingfirst transistor switching means for connecting said first statorwinding in a closed circuit across said main power supply in response toa command signal and second transistor switching means for connectingsaid second stator winding in a closed circuit across said main powersupply in the absence of a command signal, individual diodes connectedacross each of said transistor switching means and oriented to passcurrent in a direction opposite to that permitted by the associatedswitching means, an auxiliary power supply for energizing saidcommutating means, and means for interrupting the flow of power fromsaid main power supply without disturbing the flow of power from saidauxiliary power supply, so that current flow may be induced in saidstator windings by the motion of said rotor after the motor isdeenergized.

2. The motor of claim 1 wherein said sensing means includes aphotosensor and means to illuminate said photosensor only when the rotoris aligned within said given semiconductor sector.

3. The motor of claim 2 wherein said sensing means produces a commandsignal only when the photosensor is energized and illuminated.

4. The motor of claim 3 wherein said commutating means further includesbistable means for closing one of said switches when the bistable meansis in a first stable state and the other of said switches when thebistable means is in a second bistable state. 7

5. The motor of claim 4 wherein said bistable means includes first andsecond inverting means and each of said switching means includes atransistor, said first inverting means being coupled to receive acommand signal from said sensing means, said inverting means beingcoupled to receive an output signal from said first inverting means, oneof said transistors being connected to be driven into conduction by ahigh-level signal from said first inverting means, the other of saidtransistors being connected to be driven into conduction by a high-levelsignal from said second inverting means.

6. The motor of claim 5 wherein said first stator winding is connectedin series with one of said transistors across said main power supply andsaid second stator winding is connected in series with the other of saidtransistors across said main power supply.

7. The motor of claim 6 wherein both of said stator windings and oneside of said photosensor are all connected to the same side ofsaid mainpower supply, said photosensor being further coupled to said firstinverting means.

1. A single-sensor brushless DC motor comprising a cylindrical housing,first and second stator windings arranged to establish magnetic fieldsin opposite directions along a diameter of the housing, a permanentlymagnetized rotor rotatable within said housing, a main power supply forenergizing said stator windings, sensing means energized by said mainpower supply for providing a command signal when and only when the rotoris aligned with a given semicircular sector, commutating means includingfirst transistor switching means for connecting said first statorwinding in a closed circuit across said main power supply in response toa command signal and second transistor switching means for connectingsaid second stator winding in a closed circuit across said main powersupply in the absence of a command signal, individual diodes connectedacross each of said transistor switching means and oriented to passcurrent in a direction opposite to that permitted by the associatedswitching means, an auxiliary power supply for energizing saidcommutating means, and means for interrupting the flow of power fromsaid main power supply without disturbing the flow of power from saidauxiliary power supply, so that current flow may be induced in saidstator windings by the motion of said rotor after the motor isdeenergized.
 2. The motor of claim 1 wherein said sensing means includesa photosensor and means to illuminate said photosensor only when therotor is aligned within said given semiconductor sector.
 3. The motor ofclaim 2 wherein said sensing means produces a command signal only whenthe photosensor is energized and illuminated.
 4. The motor of claim 3wherein said commutating means further includes bistable means forclosing one of said switches when the bistable means is in a firststable state and the other of said switches when the bistable means isin a second bistable state.
 5. The motor of claim 4 wherein saidbistable means includes first and second inverting means and each ofsaid switching means includes a transistor, said first inverting meansbeing coupled to receive a command signal from said sensing means, saidsecond inverting means being coupled to receive an output signal fromsaid first inverting means, one of said transistors being connected tobe driven into conduction by a high-level signal from said firstinverting means, the other of said transistors being connected to bedriven into conduction by a high-level signal from said second invertingmeans.
 6. The motor of claim 5 wherein said first stator winding isconnected in series with one of said transistors across said main powersupply and said second stator winding is connected in series with theother of said transistors across said main power supply.
 7. The motor ofclaim 6 wHerein both of said stator windings and one side of saidphotosensor are all connected to the same side of said main powersupply, said photosensor being further coupled to said first invertingmeans.